OpenStax-CNX module: m20447
1
(Untitled)ACTIVITY: To describe the
composition of substances in terms
∗
of chemical symbols and formulas
Siyavula Uploaders
This work is produced by OpenStax-CNX and licensed under the
†
Creative Commons Attribution License 3.0
1 NATURAL SCIENCES
2 Matter, measuring and reactions
3 EDUCATOR SECTION
4 Memorandum
Water:
• H2 O
• Hydrogen and oxygen
• The elements oxygen and hydrogen joined. This formed a new substance, namely water, that consists
of two types of atoms. The atoms joined to form a water molecule.
• Yes, the elements are combined in a xed relation to form a new substance with new characteristics.
Figure 1
∗ Version
1.1: Mar 16, 2009 3:45 am -0500
† http://creativecommons.org/licenses/by/3.0/
http://cnx.org/content/m20447/1.1/
OpenStax-CNX module: m20447
2
Assignment 2:
1.
• 500 oxygen atoms
• 1 000 oxygen atoms
2. Combination
FORMULA
COMBINATION
O2
Oxygen molecule: two oxygen atoms
NaCl
Sodium chloride (table salt): one atom sodium and one atom chloride
N2 O5
Sodium oxide molecule: two sodium atoms combine with ve oxygen atoms
Fe2 O3
Iron oxide: two iron atoms combine with three oxygen atoms
MgCl2
Magnesium chloride: one magnesium atom combines with two chloride atoms
CO
Carbon monoxide: one carbon atom binds to one oxygen atom
Table 1
5 LEANER SECTION
6 Content
6.1 ACTIVITY: To describe the composition of substances in terms of chemical symbols and
formulas [LO 2.3]
When we know the physical and chemical properties of a substance we can use our knowledge to create
another substance. Water, a compound, is the best solvent because many substances can dissolve in water.
We can use the compound water to obtain gases like hydrogen and oxygen. We can also produce water by
mixing these gases.
We make use of chemical symbols to identify elements and use chemical formulas to identify compounds.
Here are some chemical symbols:
•
•
•
•
Ag (symbol) is used for silver
Fe is used for iron
Pb is used for lead
Au is used for gold
Try to illustrate the composition of a water molecule:
• Write the chemical formula for water.
_____________________________________________________________________
• Name the two elements that form water?
__________________________________ and _______________________________
• What had to happen to these elements to form water?
http://cnx.org/content/m20447/1.1/
OpenStax-CNX module: m20447
3
Use the words: elements, atoms and molecules in your explanation.
____________________________________________________________________
____________________________________________________________________
• Would this compound depend on a xed ratio or could it change at random? Motivate your answer.
_____________________________________________________________________
____________________________________________________________________
____________________________________________________________________
• Provide a graphic representation of the atomic composition of water:
6.2 ASSIGNMENT 2
Now try to analyse the chemical formula for carbon dioxide (carbonic acid gas):
Carbon dioxide is a compound of two kinds of atoms. The chemical formula is:
CO2
CO 2
This formula signies that one atom of carbon combines with two atoms of oxygen to form carbon dioxide.
We can therefore say that the ratio of carbon atoms to oxygen atoms in carbon dioxide molecules is 1:2.
1. How many atoms of oxygen will have to be present for:
• 250 carbon atoms?__________________________________________________
• 500 carbon atoms?__________________________________________________
2. Work out the combinations that are present in the following formulas:
FORMULA
COMBINATION
O2
_______________________________________
NaCl
_______________________________________
N2 O5
_______________________________________
Fe2 O3
_______________________________________
MgCl2
_______________________________________
CO
_______________________________________
Table 2
7 Assessment
The learner will know and be able to interpret and apply scientic, technological
and environmental knowledge.
Assessment Standard 2.3: We know this when the learner interprets information by identifying key
ideas in text, nding patterns in recorded data, and making inferences from information in various forms
(e.g. pictures, diagrams, text).
Learning Outcome 2:
http://cnx.org/content/m20447/1.1/